Atmospheric particles play an important role in atmospheric processes and human health. Indeed, particles can absorb or reflect solar radiation and can act as cloud condensation nuclei (CCN) (Charlson, R. J., Schwartz, S. E., Hales, J. M., Cess, R. D., Coakley, J. A., Hansen, J. E., and Hofinann D. J. Science, 255, 423-429 (1992); Novakov, T. and Penner, J. E. Nature, 365, 823-826 (1993)). Furthermore, particles can become trapped in different regions of the human respiratory tract and have been implicated in premature death, difficult breathing, aggravated asthma, increased hospital admissions, and increased respiratory problems to children (Seaton, A., MacNee, W., Donaldson, K. and Godden, D., Lancet, 345, 176-178 (1995)). Because of these undesirable characteristics, measurement of particle concentrations has become increasingly valuable as a regulatory tool as well as for determining health hazards.
The magnitude of the impact of airborne particles on both atmospheric processes and human health is influenced by the size distribution of these particles. Longer lifetimes and higher optical extinction efficiencies of fine aerosol (aerodynamic diameter dp less than 2.5 xcexcm) compared to coarse particles (2.5 less than dp less than 10 xcexcm) are the main reasons for the direct and indirect effects of airborne particles on climate changes (Seinfeld, J. and Pandis, S., Atmospheric Chemistry and Physics: From Air Pollution to Climate Changes, John Wiley and Sons, London, England (1997)). Moreover, exposures to particles with diameters less than 2.5 xcexcm have special importance, due to the findings of epidemiological and clinical studies, which showed a relationship between ambient particle concentration and increased respiratory problems and mortality ratesxe2x80x94(Utell, M and Samet, J., Airborne particles and respiratory disease: Clinical and pathogenetic considerations, in Particles in our Air, Wilson R, and Spengler J. D., Eds., Harvard University Press (1996)). Thus, the ability to accurately measure concentrations of particles within specific size ranges is of considerable importance.
Conventional inertial impactors have been used to classify ambient particles according to their diameter (Pierce, R. C. and Katz, M., D, Environ. Sci. Technol., 9, 347-353 (1975); Milford, J. B. and Davidson, C. I., J Air. Poll. Control. Assoc., 35, 1249-1260 (1985); Venkataraman, C., Lyons, J. M. and Friedlander, S. K., Environ. Sci. Technol., 28, 555-562 (1994)). The performance of conventional inertial impactors has been studied extensively, and their behavior and characteristics can be predicted quite accurately (Marple, V. A. and Liu, B. Y. H., Environ. Sci. Technol., 8, 648-654 (1974); Marple, V. A., Rubow, K. L., and Olson, B. A., Aerosol Measurement, Willeke, K. and Baron, P. A. Eds., Van Nostrand Reinhold, New York, 106-232 (1993)). The appropriate type of the impaction substrate depends on the species and chemical analysis to be performed.
Other types of samplers have been designed and developed. One such sampler is the virtual inertial impactor. One limitation of virtual impactors is the lack of complete separation of particles for sizes below the cut point, resulting in a mixture of concentrated coarse particles and unconcentrated fine particles in the minor flow of the impactor (Chen, B. T., Yeh, H. C. and Cheng, Y. S., Areosl. Sci. Technol., 5, 369-376 (1986)). A different type of conventional impactor with a rotating stage design, the microorifice uniform deposit impactor, has also been developed, (MOUDI) (Marple, V. A., Rubow, K. L. and Behm, S. M., Areosl. Sci. Technol., 14, 434-446 (1991)). With the MOUDI, it is still possible for bounce-off and re-entrainment losses to occur, since several tens of layers of particles are accumulated during sampling. Furthermore, multiple jet interactions can deteriorate the performance of the impactor, affecting both the cut-off point and internal losses (Fang, C. P., Marple, V. A. and Rubow, K. L. J. Aerosol.Sci. 22, 403-415 (1991)).
Many of these types of impactors and substrates used for collection of ambient particles (e.g., solid flat plates and thin porous membranes (generally 0.2 min or thinner)) exhibit decreased particle collection efficiency under such conditions. The decreased particle collection efficiency is a result of at least two factors: particles of high momentum impact the substrate and bounce off, and particles which have been previously collected are displaced from the substrate and re-entrained in the airstream (Sehmel, G. A., Environ. Intern., 4, 107-127 (1980); Wall, S., John, W., Wang, H. C. and Coren, S. L., Areosl. Sci. Technol., 12, 926-946 (1990); John, W., Fritter, D. N. and Winklmayr, W., J. Aerosol. Sci., 22, 723-736 (1991); John, W. and Sethi, V., Aerosol Sci. Technol., 19, 57-68 (1993)). In addition, because these two processes typically depend on particle size, the size distribution of the collected particles can be substantially distorted, which can confound the results of later physicochemical and biological tests performed with the collected particles.
To overcome these problems, porous metallic or glass materials have been used as impaction substrates after having been saturated with mineral oil. However, the collected particles were contaminated by substances present in the mineral oil (Reischl, G. P. and John, W., Stuab. Reinhalt. Luft, 38-55 (1978); Pak, S. S., Liu, B. Y. H. and Rubow, K. L., Aerosol. Sci. Technol., 16, 141-150 (1992); Tsai, C. J. and Cheng, Y. H., Areosl. Sci. Technol., 23, 96-106 (1995); Biswas, P. and Flagan, R. C., J. Aerosol. Sci., 19, 113-121 (1988)). The use of oil or grease-coated substrates is an important limitation for collection and analysis of ambient particles in two ways. First, the collection efficiency of these surfaces, as a function of particle size, depends on the amount of particles collected. Thus, the collection efficiency changes during the collection of a sample, as the amount of the material collected increases with time. Second, particles collected on the impactor substrate are contaminated by components of the coating material, prohibiting or interfering with certain types of chemical analysis and toxicological testing.
An impaction substrate capable of more selectively and reliably trapping particles of interest for measurement under a wide range of conditions is critical for ensuring accurate regulation, monitoring, and risk calculation.
The present invention concerns devices and methods which utilize porous substrates for the collection of particles in a gas sample. The porous substrates are useful as impaction substrates in particle collection devices such as conventional inertial impactor systems. Specifically, the substrates of the present invention can be used to collect particles of a particular size (aerodynamic diameter) range from a gas sample for analysis. They can also be used to remove particles above a given size range to allow for analysis of the particles remaining in the gas sample.
Porous substrates of the present invention offer several advantages over other materials used for the collection of particles. For example, porous substrates are capable of highly efficient particle collection, even under conditions of heavy particle loading. The use of porous materials as impaction substrates also eliminates contamination problems associated with mineral oil-coated substrates, as the porous substrates described herein are uncoated. This characteristic simplifies recovery of particles from the substrate for both chemical analysis and toxicological studies. In addition, the significantly higher collection efficiency and capacity of porous substrates allows for longer sampling time periods without significant distortion of the size distribution of collected particles.
Furthermore, because the porous materials are themselves chemically inert, the collected particles are not contaminated by the substrate and are suitable for physico-chemical and biological testing for effects on respiratory health. In contrast, use of a metal plate may result in contamination by trace metals from the plate itself, and glass fiber filters typically contain substantial amounts of trace elements which can also be a source of contamination.
Another advantage of porous substrates of the present invention is that particles can be collected on a much smaller amount of substrate material, so that it is much easier to recover the collected particles, for characterization and use for toxicological testing.
Thus, in a first aspect, the invention relates to a method of collecting particles in an accelerated gas sample by impacting particles in said gas sample on a porous material. The porous material may include foam, such as polyurethane, or cloth, such as polyester. The porous material may be at least 0.2 mm thick, preferably at least about 1 mm, or even at least about 2 mm thick. The porous material may be substantially free of oil. Additionally, the method may include passing said gas sample through an acceleration nozzle, e.g., a round or slit-shaped nozzle, prior to impacting on said porous material. The method may also include passing said gas sample through a size-selective inlet prior to impacting particles. Additionally, the method may include measuring the quantity or composition of particles deposited on said impacted on said porous material and/or measuring the composition of the airstream which flows past the impaction substrate.
In another embodiment, the invention relates to a method for sampling particles of a particular size range in a gas sample by passing said gas sample through a size-selective inlet to remove particles above a predetermined upper size from said gas sample; passing said gas sample through an acceleration nozzle; and collecting particles which pass through said acceleration nozzle on a porous impaction substrate. The porous impaction substrate may include foam, such as polyurethane, or cloth, such as polyester.
In another aspect, the invention relates to a particle sampler, such as a conventional inertial impactor, having an impaction substrate comprised of a porous material. The porous material may include foam, such as polyurethane, or cloth, such as polyester. The particle sampler may include an acceleration nozzle disposed adjacent to said impaction substrate, and/or a size-selective inlet configured to remove particles above a predetermined size from an airstream before the airstream passes over the impaction substrate.
In another embodiment, the invention provides an inertial impactor having an acceleration nozzle adjacent to an impaction substrate including a porous material as described above disposed adjacent to the acceleration nozzle. The impactor may also include a size-selection inlet configured to remove particles above a predetermined size from an airstream before the airstream passes over the impaction substrate.
In yet another embodiment, the invention relates to an inertial compactor having a sample inlet for receiving a stream of gas, a housing coupled to said sample inlet, an acceleration nozzle mounted within said housing to increase the velocity of the stream of gas, and an impaction substrate comprising a porous material as described above disposed adjacent to said acceleration nozzle to collect particles from said stream of gas. The impactor may further include a size-selective inlet mounted within said housing. Preferably, a size-selective inlet precedes a acceleration nozzle, which in turn precedes the impaction substrate, as an airstream passes through an impactor.
In still another embodiment, the invention provides a multistage gas sampling system comprising a plurality of impaction substrates configured such that a stream of air will pass over the impaction substrates in a series, wherein at least one of said substrates comprises a porous material as described above. The multistage system may include a size-selective inlet, an acceleration nozzle, and/or any other component as described above. Preferably, for each stage of the multistage system, an impaction substrate with an adjacent acceleration nozzle is present.
In yet another aspect, the invention provides a method of manufacturing an inertial impactor by providing a housing, mounting within said housing an acceleration nozzle, and mounting adjacent to said acceleration nozzle an impaction substrate comprising a porous material as described above. The method may additionally include mounting within said housing a size-selective inlet for removing particles above a predetermined size.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.